<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="jquery-3.2.0.min.js"></script>
    <script>
        $(function(){
            $('.btn p')
        })
    </script>
</head>
<body>
<div id="box" class="btn">
    <p></p>
</div>
</body>
</html>